The present invention relates in general to semiconductor devices, and, in particular, to a new and useful method of fabricating a semiconductor device using high temperature brazing to connect contact members to the semiconductor material, while avoiding contamination of the semiconductor junction in the semiconductor material by metal ions.
Semiconductor devices have been manufactured by brazing semiconductor dies between two contact members. These contact members may be disks, slugs or leads. Subsequent to connecting the contact members, a chemical etching process is performed to improve the electrical characteristics of the device. This is followed by glass encapsulation using various methods such as coating with glass slurry, firing or melting a glass sleeve around the semiconductor junction, to protect the junction from the environment and to stabilize the electrical characteristics of the device.
To facilitate connecting the device to other components in an electrical circuit, connecting members are usually attached to the contact members. The connecting members may be in the form of wires, metal disks or slugs.
The use of high temperature brazing joints between the semiconductor die and the contact members helps provide a strong and stable structure.
A major problem associated with such a construction process is that during the chemical etching step, the metal contact and connecting members are etched along with the semiconductor material. Metal ions are deposited on the semiconductor rectifying junction in minute quantities. This causes contamination of the semiconductor junction which results in deterioration of its electrical properties. In particular, the peak reverse voltage and reverse leakage currents are adversely effected. Another effect is increasing instability of the electrical characteristics of the semiconductor device, both short term and long term.
To reduce such contamination during the etching process, techniques have been developed to etch and pre-passivate the semiconductor junctions during the wafer stage. This is before the application of contact metalization and before dividing the wafer into single individual semiconductor dies. Pre-passivation is often done by applying several layers of glass directly on the junction of the semiconductor. The glass layers may also be applied with intermediate layers consisting, for example, of oxides, nitrides, polysilicon, or other combinations.
After division or singulation of the wafer, pre-passivated semiconductor units are obtained. These are sometimes referred to as glass passivated pellets or GPP's.
Among the known methods of applying glass in the process of manufacturing the glass passivated pellets are spin coating, followed by photolithography, doctor blading, electrophoresis and screen printing. GPP's are then soldered to contact members, using soft low temperature solders with melting temperatures below 350.degree. C. and containing mostly such metals as lead, tin and indium, with other small additives.
Semiconductor devices using soldered GPP's do not always have sufficient mechanical strength, however.
Another limitation of soft-soldered devices is thermal fatigue of the solder, and, generally, much worse reliability at elevated temperatures than the brazed devices. Those limitations were long recognized, but attempts to braze the GPP dies were unsuccessful due to severe degradation of electrical characteristics occurring during brazing, in particular, significant increase of forward voltage drop and catastrophic decrease of peak reverse voltage.